On the generalized Chebyshev polynomials corresponding to plane trees of diameter 4

The combinatorial classification of plane trees by the number of realizations of their valency sets has distinguished some special classes of plane trees. One of them, the plane trees of diameter 4, turned out to be a very interesting object of investigation from the Galois action point of view. In this paper, we present equation sets for some subclasses of trees of diameter 4, calculate discriminants of the corresponding generalized Chebyshev polynomials, some related polynomials, and their fields of definitions, and use this to get some information about the Galois action on plane trees.